The coating of thermally stressed parts of gas turbines or burners equipped for the cooling of their walls with cooling holes for the air or gas cooling usually cannot be renewed easily by using overlay coating and/or thermal barrier coating, TBC, because the cooling holes are at least partially closed or constricted by the repeat coating, so that the cooling effect is changed and/or adversely affected. This is true, in particular, if the cooling holes have been provided in the wall after the original coating.
Several solutions have been described for preventing harm from constrictions formed when coating parts provided with cooling holes. In one type of solution, measures are taken to prevent the constrictions from even forming. To accomplish this, the cooling holes are closed or filled prior to the coating. The filling of the cooling holes prevents coating material from penetrating into the holes. After the coating process is complete, the filling is again removed from the cooling holes, and the cooling holes are available with their original through-cross-section. Such techniques are known, for example, from U.S. Pat. No. 4,743,462 and U.S. Pat. No. 5,800,695.
In another proposed solution, the cooling holes are left open, and the constrictions formed in the holes during coating are removed after the coating, by using a pluse UV laser beam as is known from U.S. Pat. No. 5,216,808. It would also be conceivable, however, to remove the constrictions by pressing a grinding fluid through the cooling holes (U.S. Pat. No. 5,702,288) that abrades the constrictions in this manner.
The disadvantage of this method is that the individual cooling holes must be carefully closed and then again exposed or freed from the constriction again carefully so that the originally intended cooling effect is fully preserved at all places on the cooled part, and no local overheating occurs as a result of insufficiently removed constrictions. In parts equipped with a large number of finely distributed cooling holes, this creates significant work during the processing.